The present invention relates generally to an apparatus for the detection and measurement of particulates in molten metal and, more particularly, to an improved apparatus which includes a liquidus depressing material which lowers the temperature at which sampled molten metal would normally begin to solidify, thereby, allowing for more molten metal to be sampled over a longer time period for enhanced particulate measurement.
Molten metals, particularly molten aluminum and steel, are frequently contaminated to some extent by entrained non-metallic inclusions that potentially give rise to a variety of shortcomings or defects in the resulting finished metal products. More often, a certain size or size range of non-metallic inclusions, such as alumina in deep drawing stock, is known to be harmful to the performance of the drawing stock. Knowledge of the quantity of such inclusions would be beneficial in determining the fitness for service of the finished product.
An apparatus for electrical zone sensing of suspended particles in a liquid is disclosed in U.S. Pat. No. 2,656,508 issued Oct. 20, 1953 to Wallace A. Coulter. In a typical apparatus, a tube having an aperture in its wall is positioned within a larger vessel. A liquid electrolyte suspension containing the particles to be detected and measured is placed in the vessel and is induced to flow into the tube through the aperture by establishing a fluid pressure differential between the interior of the tube and the vessel. The vessel and the tube are both fabricated of an insulator, e.g. glass, and a constant electric current is placed across the aperture. The presence of a particle in the liquid flowing through the aperture causes a change in the electrical resistance detected at the aperture and the electric voltage producing the constant current varies directly with the resistance change each time a particle passes through the aperture. A detecting circuit determines the size of the particles passing through the aperture from the change in resistivity caused by each particle, this depending upon the volume of electrolyte at the aperture displaced by the particle and by the resistivity of the kind of particles. The information is amplified and processed by suitable electronic circuits.
U.S. Pat. No. 4,555,662 describes a method and apparatus for the detection and measurement in a molten metal sample of suspended particulates of greater than a predetermined size whose electrical conductivities differ from that of the suspending molten metal. The apparatus comprises an electrically insulating vessel having a small passage (typically 200 to 500 microns in diameter) extended therethrough; a pair of electrodes disposed within and outside of the vessel to establish a current path between them through the molten metal of the sample and passing through the small passage; means for passing a sample of the molten metal through the passage; and means for passing an electric current between the two electrodes through the molten metal in the current path and for detecting a voltage change resulting from the flow of particulates through the passage. The apparatus also includes means for counting the number of voltage changes during a particular measuring period as representative of the number of particulates, and for measuring the magnitude of each of the voltage changes as representative of the size of the particulates causing the changes. The device described comprises a refractory tube with the small hole at its lower end, which is dipped into the molten metal, for example in a trough along which the molten metal is flowing. One electrode is positioned within the tube and the other outside of the tube. Molten metal is caused to pass through the small hole by means of a differential pressure applied to the tube.
The principle of operation of the apparatus described in the two above-identified patents generally refers to the measurement of non-metallic particles in molten aluminum. The devices used for the particulate measurement in molten aluminum are unsuitable for use in molten steel due to the large difference in the respective processing temperatures. The described particle counters commonly evaluate molten aluminum at a temperature of around 750xc2x0 C., however, the temperature of a steel measurement would be closer to about 1550xc2x0 C. Simple substitution of more suitable materials for the components of the apparatus cannot be assumed. The availability of materials which are capable of withstanding such high temperatures and are stable at such temperatures for the relatively long periods of time needed to make meaningful particulate measurements are limited as well as very expensive.
U.S. Pat. No. 5,198,749 attempts to address the numerous differences in apparatus construction due to the high processing temperature of steel and its alloys and provide a measuring strategy to overcome the difficulty of relatively long measurement times at high temperatures. The device of the ""749 patent comprises a single use disposable probe that is detachably connected to a support member. The detachability of the device is common to those skilled in the art of disposable sensors for the molten iron and steel industry. The probe comprises electrode and orifice configurations of the prior more continuous devices and a jet limiting insert which serves to help cool the incoming metal immediately upon immersion of the probe into the molten metal. A meltable cover closes the orifice prior to immersion of the probe and the cover is protected by a meltable shield (slag cap) enabling the probe to be passed through an overlying slag layer without entry of slag into the probe interior. Such capping is also well known to those skilled in the art of disposable molten metal sensors. The filling of the inner chamber with the molten metal may be assisted by a reduced pressure established within the tube, or may be slowed by a positive pressure to maintain the Reynolds number of the flow below 2000. The inner chamber, is divided by a narrow bore into two compartments so that when metal enters and fills one compartment it will freeze in the bore so that it cannot enter the second compartment, protecting the vacuum source, if provided, and establishing a prescribed quantity of metal entering the probe.
Although the device of the ""749 patent uses the principle of disposable, short term measurements of approximately 2 minutes to overcome the problems of long term high temperature measurements, the solution of the ""749 patent has introduced a new set of problems. A short term measuring device of the above-described construction does not provide suitable time for preheating of the internal components of the probe. The entering molten metal is cast against the interior probe material that is close to room temperature and is quickly cooled. The lack of suitable preheating results in premature solidification of the molten metal entering the inner chamber effectively limiting the amount of metal which may be sampled. The liquidus temperature of a molten material is the temperature at which a solid phase begins to precipitate from the cooling liquid. The difference between the molten metal processing temperature and the liquidus temperature is called the superheat. An additional problem arises when such probes are intended for immersion in a tundish of molten steel during continuous casting. The temperature of molten steel in the tundish is generally on the order of 20-40xc2x0 C. above the liquidus temperature of the steel, providing a super heat 20-40xc2x0 C. The liquid steel possesses a low heat content and an inability to raise the temperature of the inner chamber walls of the probe so as to maintain a non-freezing sampling condition. The mass of the sampling apparatus itself chills the liquid metal in the chamber during filling by thermal conduction to cooler portions of the probe, thus limiting the useful application of such probes to metals having a suitable super heat.
In accordance with the present invention there is provided a molten metal inclusion sensor of the disposable type that is immersed into molten metal at a temperature near its solidification temperature, a low superheat application, and detects inclusions in the molten metal by the electric sensing zone method of the prior art. The present invention is characterized by a probe which is immersed in the molten metal for detection of the inclusions, the probe having a highly heat insulating arrangement of the inner chamber and the inner chamber containing one or more additives that effectively lower the liquidus or solidification temperature of the entering metal.
The present invention comprises an apparatus for the detection of non-conductive particulates in a bath of molten metal, specifically steel and alloys of high iron content. In use, molten metal is pumped through an orifice in an electrically insulating refractory wall to establish a current path from an inner container through the orifice to the bath of molten metal. A current is passed along the current path. Voltage changes, in the form of pulses, are measured as indicating passage of suspended particulates through the orifice. The size of the pulses provides an indication of the particle size and counting the number of pulses gives the size distribution of detected non-metallic inclusion in the molten metal.
The present invention provides a disposable apparatus for the detection and measurement of the concentration and size distribution of suspended particulates in molten metal by the electric sensing zone method that is operative relatively rapidly and has a minimum sensor mass. In one embodiment, the internal chamber electrode(s) are located along the chamber wall so that metal entering the chamber and xe2x80x9cwellingxe2x80x9d up the thermal center of the chamber is not prematurely cooled by the electrode. As the metal fills the chamber it flows up through the thermal center then contacts the walls and electrode where it freezes and solidifies. The present invention can be employed during a processing operation on the molten metal and is capable of measuring metal close to its solidification temperature. The present invention includes an additive for decreasing the liquidus temperature of the metal in the inner chamber by alloying the entering metal with another selected metal, or selected metals chosen from several metals which are known to lower the liquidus temperature of the entering metal effectively increasing the apparent superheat and permitting a longer effective measuring time for the probe.
Briefly stated, the present invention comprises an improved probe for insertion into molten metal to detect and measure particulates suspended therein using the electric sensing zone method. The probe comprises a generally sealed inner tube of an electrically insulated material forming a molten metal receiving chamber. The tube includes at least one orifice proximate an insertion end of the probe to permit molten metal to flow into the chamber. A first electrode extends into the chamber for engaging metal within the chamber. A second electrode surrounds at least a portion of the inner tube for engaging molten metal outside of the chamber. The first and second electrodes are connectable to a measurement device for establishing a current path through the electrodes and passing through the orifice and for measuring changes in the electrical potential between the electrodes produced by the passage of particulates entrained in the molten metal passing through the orifice. An outer sheath of heat resistant material surrounds at least a portion of the second electrode to provide thermal insulation therefore. A gas passageway extends out of the inner tube for connecting to a vacuum source to create a pressure differential between the inside and outside of the inner tube for facilitating the flow of molten metal through the orifice. The improvement comprises a liquidus depressing material installed within the chamber for alloying with molten metal entering the chamber to lower the liquidus temperature of the molten metal in the chamber and permit a longer time period for detecting and measuring particulates in the molten metal.